The long-term goal of this study is to better understand how vestibular and somatosensory information are integrated to control human postural stability. The specific aims of this proposal are: 1) to determine how bilateral loss of vestibular function affects postural responses to surface and vibratory perturbations; 2) to determine whether somatosensory disruption affects postural responses to head and vestibular perturbations; 3) and to characterize how vestibular and somatosensory inputs interact in the control of postural stability. Although vestibular and somatosensory loss are known to cause severe postural instability, the relationships between these sensory systems in normal and disordered postural control are not well understood. Understanding how these sensory systems interact together is important because multisensory deficits involving both the vestibular and somatosensory systems are the most common etiology for "nonspecific" balance problems in the elderly, affecting the majority of people over 70 years old. Compensating for vestibular or somatosensory loss depends upon complex interactions of remaining senses in the central nervous system, yet there are few studies focused on these sensory interactions in humans. We hypothesize that: 1) the loss of peripheral vestibular function in patients with vestibular disorders results in an increased gain of postural responses elicited by somatosensory inputs; 2) that disruption of somatosensory function in patients with diabetic peripheral neuropathy or in healthy subjects standing on altered surfaces results in increased gain of postural responses elicited by vestibular inputs; and 3) that combinations of vestibular and somatosensory perturbations to posture interact in complex ways in healthy subjects. Somatosensory perturbations to stability will be induced mechanically with a moveable platform and directly by muscle vibration. Vestibular perturbations will be induced mechanically with an air-jet head perturber and directly with galvanic stimulation to the mastoid processes. Postural responses of the legs, trunk, and head will be quantified by muscle activation patterns, surface reactive forces under each foot, and kinematics of body motion. The influence of vision will be eliminated by blindfolding all subjects. These studies will provide insight into the interactive mechanisms underlying effective compensation for vestibular and somatosensory deficits. A better understanding of vestibular-somatosensory interactions will allow us to better identify abnormal sensory interactions in patients with balance problems and to develop effective approaches to facilitate sensory compensation.